Fiber reinforced plastic (FRP) pipes have been heretofore manufactured by impregnating a reinforcing material such as inorganic fibers (e.g., glass fibers and carbon fibers) or organic fibers (e.g., aramide fibers) with an uncured thermosetting resin, a thermoplastic resin or other resin as a matrix, charging a mold with these starting materials, drawing the starting materials, or extruding the starting materials, to make them into a shape of a pipe.
Of such starting materials as mentioned above, carbon fiber reinforced plastics (hereinafter abbreviated to "CFRP") excellent in specific strength and specific modulus of elasticity have been predominantly employed in recent years. As the matrix resin, thermosetting resins are used in most cases because of their excellent heat resistance, water resistance, solvent resistance, chemical resistance, dimensional stability, etc., and among these resins, epoxy resins, which show high bond strength to carbon fibers, are mainly used as the matrix.
A prepreg obtained by impregnating carbon fibers with the epoxy resin and subjecting the resin to the B-stage procedure (i.e., drying a liquid thermosetting resin to polymerize it to a certain degree) is commercially available. This prepreg is wound around a mandrel and heat cured to form a pipe. The pipe can be also formed by winding a bundle of fibers (strands or filaments) impregnated with the epoxy resin around a mandrel at a certain angle through filament winding and then curing the resin. The pipes formed by the above methods can be per se used. However, if the pipes need surface smoothness, or painting is required to be effected on the pipe surface, or the pipes need roundness required for use as a high-speed rotator, they are often subjected to surface abrading treatment.
Because of the surface abrading treatment, both the carbon fibers and the resin are exposed outside on the pipe surface, and this results in various problems. For example, the carbon fibers split finely on the pipe surface in the course of the abrasion to produce surface defects. Further, since the both ends of the pipe cut after removal from the mold are rigid and fragile, the corner portions of those ends are easily dropped even by slight impact applied to the pipe during the abrasion, and depending upon the structure of layer lamination of the reinforced fibers, even breakage of the pipe occurs frequently.
For coping with these problems, there has been made a trial that the pipe surface is made to be a uniform resin surface free from exposure of any carbon fibers thereby to render the surface strong enough to withstand impact of a certain level. In the conventional process for manufacturing a pipe using a mandrel, however, a prepreg or resin-impregnated filaments are wound around the mandrel, and then a shrinkable tape is further wound tightly around the prepreg or filaments to allow the resin to ooze out, followed by curing. Therefore, this process involves such a problem that the resin cannot be hardly left in a relatively large thickness on the pipe surface.
In order to manufacture a FRP pipe coated with a resin, accordingly, generally adopted is a method wherein operations consisting of coating the surface of the FRP pipe with a resin and curing the resin are repeated to form a resin surface layer having a predetermined thickness.
Also known is a method of encasing a FRP pipe in a heat-shrinkable tube such as a Teflon tube and heat shrinking the tube to coat the pipe surface in order to impart releasability to the surface of the FRP pipe, or a method of forming a coat of modified PPE (polyphenylene ether) on the surface of the FRP pipe through co-extrusion to manufacture a composite pipe in order to improve appearance of a glass fiber reinforced, modified PPE pipe.
These methods, however, involve various problems. For example, the number of the steps is made large to cause increase of the cost, the bond strength between the inner surface of the Teflon shrinkable tube and the outer surface of FRP is low to cause separation therebetween in the use of the resulting pipe, or the PPE is a thermoplastic resin having low bond strength to the carbon fibers. Thus, any simple and easy technique for manufacturing a FRP pipe provided with a resin coat of sufficient thickness as the outer layer has not been developed yet.
In the use of the FRP pipe, the surface of the pipe is frequently subjected to metal coating treatment to cope with static electrification on the surface or to improve surface hardness and abrasion resistance. Accordingly, various proposals have been made with respect to the surface metal coating.
For example, there is known a method comprising abrading a CFRP pipe to expose the resin and the carbon fibers on the surface of the pipe and coating the abraded surface with copper through electroplating utilizing the conduction properties of the carbon fibers. According to this method, however, the bond strength between the CFRP surface and the plated copper is low, e.g., about several tens g/cm, and hence lifting takes place in the course of the plating or in the course of the abrasion after formation of the plated layer.
In order to compensate such low bond strength, the copper-plated layer is made thick, usually about 500 .mu.m. However, formation of such thick plated layer deteriorates the lightweight properties of the pipe given by the use of CFRP, and in the use of the pipe as a rotator, the moment of inertia becomes large, and therefore the inherent performance of the CFRP is not sufficiently exerted.
Japanese Patent Publication No. 12541/1991 discloses technique for increasing the bond strength between the plated copper and the CFRP, which comprises coating the abraded surface of the CFRP pipe with a resin added with an Ag powder, curing the resin to form a conductive resin layer and effecting electroplating utilizing the conduction properties of the resin layer. Japanese Patent Laid-Open Publication No. 124278/1987 discloses technique comprising adsorbing metallic colloid on the FRP prior to the plating operation and effecting electroless plating to impart conduction properties to the CFRP.
Even in the above technique, however, the bond strength and the conduction properties are not sufficiently improved, or rather, the process is complicated and the cost is increased.
By the way, thermoplastic resins such as an ABS (Acrylonitrile-Butadiene-Styrene) resin and a polypropylene resin can be generally provided with a metal-plated coat showing high bond strength to the resins through electroless plating.
However, the CFRP which contains as a matrix the thermoplastic resin capable of being plated through electroless plating is low not only in the bond strength between the carbon fibers and the resin but also in the heat resistance, and moreover tends to be distorted by thermal expansion or heat shrinkage owing to the environmental temperature given when the CFRP is molded into a pipe or thereafter. Hence, it is difficult to manufacture a pipe of high dimensional stability.
The present inventors have earnestly studied on such problems as mentioned above, and as a result, they have found that a FRP pipe having high bond strength between its inner and outer layers and improved in impact resistance can be obtained by a process comprising the steps of winding an uncured B-staged prepreg obtained by impregnating reinforcing fibers with a thermosetting resin to form an inner layer, winding a thermoplastic resin sheet or tape around the inner layer to form an outer layer so as to unite those two layers, and heat curing the inner layer while thermocompression bonding the outer layer to the inner layer. The present inventors have also found that a FRP pipe having a metal coat, which is lightweight, excellent in surface hardness and abrasion resistance and has attained thinning of the metal-plated layer, can be manufactured by using, as a resin for forming the outer layer, a thermoplastic resin capable of being heightened in the bond strength to the metal-plated layer obtained by electroless plating. Based on these findings, the present invention has been accomplished.